1. Field of the Invention
This invention concerns a discharge lamp used as a point light source.
2. Description of Related Art
In recent years, liquid crystal projectors have been used as a presentation tool in conferences, exhibitions and so on. A liquid crystal screen can be projected onto a projection screen by means of a high-intensity light source, but in the past the high-intensity light sources used for liquid crystal projectors have been very high-pressure mercury lamps or metal halide lamps which have a pair of facing electrodes within a bulb of silica glass and a given light-emitting substance sealed into the glass bulb. Such lamps are then sealed closed using a foil seal or a rod seal.
Recently, however, there has been increasing market demand for liquid crystal projectors of greater brightness, and consequent demand for brighter light sources for that use. Most recently, very high-pressure mercury lamps with a high sealing pressure have been taking over the leading role from metal halide lamps. However, because there are limits to the pressures that can be withstood by the seals of very high-pressure mercury lamps sealed with foil seals, they are expected to reach their limits for increased intensity within the near future.
Therefore, non-electrode lamps that do not have foil seals are conceivable, in terms of pressure resistance, as alternative light sources for projectors. However, the type of discharge has been considered is the tube-stabilized discharge type which requires forced cooling because the arc discharge follows the tube wall of the discharge vessel and imposes a thermal load on the tube walls of the discharge vessel. Moreover, the arc discharge cannot be confined to the center of the lamp, and is completely unsuitable as a point light source.
Accordingly, a lamp with the structure shown in Japanese Patent HEI-225744 (1991) was proposed as a light source without a foil seal. This is a low pressure discharge lamp, and can be used for such things as back lighting in miniature liquid crystal televisions. A pair of cylindrical, metal, internal electrodes are fixed in the terminals of the discharge vessel; external electrodes are placed on the outer wall of the glass seal corresponding to the cylindrical internal electrodes, forming a condenser of the glass seal sandwiched between the external electrodes and the cylindrical internal electrodes. When a high-frequency voltage is impressed on the external electrodes, power is fed to the cylindrical internal electrodes. However, this lamp is a low-pressure discharge lamp that uses the ultraviolet radiation generated by discharge between the internal electrodes by converting it to visible light by means of a fluorescent layer on the inner wall of the discharge vessel; it cannot be used as a point light source.
The primary object of this invention, therefore, is to provide a lamp device that is a point light source, that can withstand high pressure, and that can produce high-intensity light.
The above object of this invention is achieved by a high-frequency excitation point light source lamp device comprising: a discharge vessel made of a transparent, non-conductive material and having an expanded part with tubules joined to it; a lamp having a discharge concentrator that concentrates the electrical field within the discharge space of the expanded part, the tips of which are supported by the tubules and face each other within the discharge space; and a means external to the lamp that supplies high-frequency excitation energy that excites a discharge of the concentrator.
Moreover, this invention can have a high-frequency power supply as the means to provide the high-frequency excitation energy, and will be a high-frequency excitation point light source lamp device in which the discharge is excited by capacitance coupling. Or it can have a microwave source as the means to provide the high-frequency excitation energy, and will be a high-frequency excitation point light source lamp device in which the discharge is excited by electromagnetic resonance. Thus, in the event that it has a microwave source as the means to provide the high-frequency excitation energy, it will be a high-frequency excitation point light source lamp device in which the materials receiving the microwaves are placed on the outer periphery of the tubule.
Moreover, the discharge concentrator has a pair of tips facing each other within the discharge space. It is preferable that the gap between the tips of the concentrator be less than the inner diameter of the expanded part. It is also possible to have a single discharge concentrator. In addition, it is preferable that the back ends of the discharge concentrator be reduced in diameter, or that the back ends of the discharge concentrator have a curved surface. And it is preferable that the tips of the discharge concentrator be pointed.
Additionally, it is preferable to select as the material for the discharge concentrator a material having a critical temperature of use that is higher than the critical temperature of use of the nonconductive material of the discharge vessel. Moreover, it is preferable that the material selected for the discharge concentrator have lower wettability than the non-conductive material of the discharge vessel. It is also possible to select a dielectric material as the material for the discharge concentrator.
Silica glass or a transparent ceramic can be selected as the non-conductive material of the discharge vessel. It is possible to have 300 mg/cc or more mercury sealed within the lamp, or to have xenon sealed within the lamp with a sealing pressure of at least 6 MPa at 300 K. Moreover, the gap between the discharge vessel and the discharge concentrator can be filled with mercury. In the event that the high-frequency excitation energy is provided by a high-frequency power supply, it is preferable that the lamp be lit by a high frequency of at least 100 MHz.
The lamp device of this invention is constructed with a discharge vessel of a non-conductive material, and the concentrator is contained entirely within the discharge vessel. Because there is no seal where a current conductor passes to the outside of the lamp, as in the prior technology, the gas pressure that can be withstood within the lamp during discharge is high.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention.